Even if you never publish your package, you can still get a lot of
benefits of using npm if you just want to write a node program (a), and
perhaps if you also want to be able to easily install it elsewhere
after packing it up into a tarball (b).

npm install (in package directory, no arguments):

Install the dependencies in the local node_modules folder.

In global mode (ie, with -g or --global appended to the command),
it installs the current package context (ie, the current working
directory) as a global package.

By default, npm install will install all modules listed as dependencies
in package.json(5).

With the --production flag (or when the NODE_ENV environment variable
is set to production), npm will not install modules listed in
devDependencies.

npm install <folder>:

Install a package that is sitting in a folder on the filesystem.

npm install <tarball file>:

Install a package that is sitting on the filesystem. Note: if you just want
to link a dev directory into your npm root, you can do this more easily by
using npm link.

Example:

npm install ./package.tgz

npm install <tarball url>:

Fetch the tarball url, and then install it. In order to distinguish between
this and other options, the argument must start with "http://" or "https://"

When using any of the above options to save dependencies to your
package.json, there are two additional, optional flags:

-E, --save-exact: Saved dependencies will be configured with an
exact version rather than using npm's default semver range
operator.

-B, --save-bundle: Saved dependencies will also be added to your bundleDependencies list.

Further, if you have an npm-shrinkwrap.json then it will be updated as
well.

<scope> is optional. The package will be downloaded from the registry
associated with the specified scope. If no registry is associated with
the given scope the default registry is assumed. See npm-scope(7).

Note: if you do not include the @-symbol on your scope name, npm will
interpret this as a GitHub repository instead, see below. Scopes names
must also be followed by a slash.

Install the package at https://gist.github.com/gistID by attempting to
clone it using git. The GitHub username associated with the gist is
optional and will not be saved in package.json if -S or --save is used.

If you don't specify a commit-ish then master will be used.

Example:

npm install gist:101a11beef

npm install bitbucket:<bitbucketname>/<bitbucketrepo>[#<commit-ish>]:

Install the package at https://bitbucket.org/bitbucketname/bitbucketrepo
by attempting to clone it using git.

If you don't specify a commit-ish then master will be used.

Example:

npm install bitbucket:mybitbucketuser/myproject

npm install gitlab:<gitlabname>/<gitlabrepo>[#<commit-ish>]:

Install the package at https://gitlab.com/gitlabname/gitlabrepo
by attempting to clone it using git.

If you don't specify a commit-ish then master will be used.

Example:

npm install gitlab:mygitlabuser/myproject

You may combine multiple arguments, and even multiple types of arguments.
For example:

npm install sax@">=0.1.0 <0.2.0" bench supervisor

The --tag argument will apply to all of the specified install targets. If a
tag with the given name exists, the tagged version is preferred over newer
versions.

The --dry-run argument will report in the usual way what the install would
have done without actually installing anything.

The -f or --force argument will force npm to fetch remote resources even if a
local copy exists on disk.

npm install sax --force

The -g or --global argument will cause npm to install the package globally
rather than locally. See npm-folders(5).

The --global-style argument will cause npm to install the package into
your local node_modules folder with the same layout it uses with the
global node_modules folder. Only your direct dependencies will show in
node_modules and everything they depend on will be flattened in their
node_modules folders. This obviously will eliminate some deduping.

The --ignore-scripts argument will cause npm to not execute any
scripts defined in the package.json. See npm-scripts(7).

The --legacy-bundling argument will cause npm to install the package such
that versions of npm prior to 1.4, such as the one included with node 0.8,
can install the package. This eliminates all automatic deduping.

The --link argument will cause npm to link global installs into the
local space in some cases.

The --no-bin-links argument will prevent npm from creating symlinks for
any binaries the package might contain.

The --no-optional argument will prevent optional dependencies from
being installed.

The --no-shrinkwrap argument, which will ignore an available
shrinkwrap file and use the package.json instead.

The --nodedir=/path/to/node/source argument will allow npm to find the
node source code so that npm can compile native modules.

The --only={prod[uction]|dev[elopment]} argument will cause either only
devDependencies or only non-devDependencies to be installed regardless of the NODE_ENV.

See npm-config(7). Many of the configuration params have some
effect on installation, since that's most of what npm does.

ALGORITHM

To install a package, npm uses the following algorithm:

load the existing node_modules tree from disk
clone the tree
fetch the package.json and assorted metadata and add it to the clone
walk the clone and add any missing dependencies
dependencies will be added as close to the top as is possible
without breaking any other modules
compare the original tree with the cloned tree and make a list of
actions to take to convert one to the other
execute all of the actions, deepest first
kinds of actions are install, update, remove and move

For this package{dep} structure: A{B,C}, B{C}, C{D},
this algorithm produces:

A
+-- B
+-- C
+-- D

That is, the dependency from B to C is satisfied by the fact that A
already caused C to be installed at a higher level. D is still installed
at the top level because nothing conflicts with it.

For A{B,C}, B{C,D@1}, C{D@2}, this algorithm produces:

A
+-- B
+-- C
`-- D@2
+-- D@1

Because B's D@1 will be installed in the top level, C now has to install D@2
privately for itself.

See npm-folders(5) for a more detailed description of the specific
folder structures that npm creates.

Limitations of npm's Install Algorithm

npm will refuse to install any package with an identical name to the
current package. This can be overridden with the --force flag, but in
most cases can simply be addressed by changing the local package name.

There are some very rare and pathological edge-cases where a cycle can
cause npm to try to install a never-ending tree of packages. Here is
the simplest case:

A -> B -> A' -> B' -> A -> B -> A' -> B' -> A -> ...

where A is some version of a package, and A' is a different version
of the same package. Because B depends on a different version of A
than the one that is already in the tree, it must install a separate
copy. The same is true of A', which must install B'. Because B'
depends on the original version of A, which has been overridden, the
cycle falls into infinite regress.

To avoid this situation, npm flat-out refuses to install any
name@version that is already present anywhere in the tree of package
folder ancestors. A more correct, but more complex, solution would be
to symlink the existing version into the new location. If this ever
affects a real use-case, it will be investigated.